The present invention relates to a direct digital synthesizer (hereafter abbreviated to DDS) capable of conducting frequency switching while maintaining phase coherency, and a nuclear magnetic resonance instrument (hereafter abbreviated to NMR instrument) using such a DDS.
The NMR instrument is an instrument which transmits a high-frequency alternating current magnetic field to a molecule placed in a strong static magnetic field, gives perturbation to a nuclear spin in the molecule, and measure a response of the nuclear spin.
The alternating current magnetic field takes a pulse having a definite frequency, phase and amplitude as a basic unit. In experiments using the NMR instrument, a series of pulses which are different in frequency, phase and amplitude are transmitted to the molecule. Such a series of pulses is called pulse sequence. The pulse sequence is designed carefully by an NMR researcher with the object of controlling the nuclear spin state according to the purpose of the experiment. Its example can be seen in Non-Patent Document 1. The number of pulse sequences has already exceeded several hundreds, and it is increasing day by day with the advance of the NMR research.
A high-frequency transmitter (hereafter abbreviated to transmitter) in the NMR instrument generates high-frequency pulses changed in frequency, phase and amplitude in a time of several microseconds or less according to the pulse sequence. In the transmitter which needs fast and highly precise operation, introduction of the DDS was a great breakthrough. The DDS is an integrated circuit for signal generation and modulation using digital techniques, and the DDS can generate frequency-, phase-, and amplitude-modulated pulses at high speed and with high resolution. As regards the DDS, there is a large literature. For example, Non-Patent Document 2 is one of them. An example of a DDS capable of modulation at high speed and with high resolution as a feature is AD9956 produced by Analog Devices.
According to Non-Patent Document 3, the AD9956 can modulate a signal in a band of DC to 200 MHz with a frequency resolution of 1.42 micro hertz and a phase resolution of 0.02 degrees in a time of only 10 nanoseconds. As compared with the conventional analog circuit, the performance of the DDS is several digits higher. The conventional analog circuit has a frequency resolution in the range of 0.1 to 1 Hz and a phase resolution of several degrees, and the conventional analog circuit needs a time in the range of several tens to several hundreds microseconds for modulation.
One problem posed when utilizing the DDS in the transmitter in the NMR instrument is to maintain the phase coherency in frequency modulation. FIG. 2 shows the problem of the phase coherency. When the frequency is changed from F1 to F2 and restored to F1 as shown in A, the output waveform of the DDS changes with a continuous phase at a boundary between frequencies as shown in B. In the NMR instrument, however, phase information is important. Therefore, a waveform which maintains the phase coherency as shown in C is necessary. The phase coherency can be appreciated easily by comparing C, D and E in FIG. 2. In FIG. 2, D and E are two virtual output waveforms respectively maintaining frequencies F1 and F2. In FIG. 2, C has the waveform of D when the output frequency is F1, and has the waveform of E when the output frequency is F2.
In the NMR, it is necessary to maintain the phase coherency between pulses having the same frequency as shown in C in FIG. 2. As represented by B, however, the waveform of the DDS cannot maintain the phase coherency. One clear solution method to this problem is proposed in Patent Document 1. According to the Patent Document 1, a plurality of phase accumulators (hereafter abbreviated to PAs) are mounted on one DDS and one output frequency is assigned to each PA. Phase coherency at each frequency is maintained by the assigned PA. The output frequency of the DDS is changed by changing the PA selected to be used for the output from among a plurality of PAs. The technique disclosed in the Patent Document 1 can satisfy both the frequency modulation using the DDS and maintenance of the phase coherency.
Patent Document 1: Japanese Patent No. 3855237
Non-Patent Document 1: Stefan Berger and Siegmar Braun, “200 And More NMR Experiments: A Practical Course”, Wiley-VCH, 2004
Non-Patent Document 2: “A Technical Tutorial on Digital Signal Synthesis”, Analog Devices, 1999
Non-Patent Document 3: “AD9956 Data Sheet Rev. A”, Analog Devices, 2004
In the technique disclosed in the Patent Document 1, however, the number of frequencies which can be used in the pulse sequence is greatly restricted. In the technique disclosed in the Patent Document 1, an output frequency is assigned to each PA. Therefore, the number of output frequencies which can be used in the pulse sequence coincides with the number of PAs mounted on the DDS. Considering the present state in which the number of pulse sequences continues to be increasing day by day with the advance of the NMR research, it can be expected easily that the number of frequencies required for the pulse sequence will exceed the number of PAs within the life of the NMR instrument even if PAs which are sufficient in number to leave a margin are mounted when the NMR instrument is manufactured. In a worse case, the number of PAs mounted on the NMR instrument leads the research itself of the pulse sequence in a biased direction. In other words, there is even a fear that it might be hesitated to conduct researches in the pulse sequence using frequency modulation because the number of frequencies which can be used in the NMR instrument is small.